Graphene oxide (or graphite oxide, hereinafter GO) is a sheet-shaped carbon material prepared by acid treating graphite, and has a large amount of a hydrophilic functional group such as a carboxyl group (—COOH), a hydroxyl group (—OH), and the like on the surface. The surface oxidizing groups produced through an acid treatment process spontanesouly produce hydrogen-bonds with H2O molecules, and thus the GO can be prepared in a form of a hydration or in a water-containing slurry state. In general, a solid concentration of the slurry is of about 2 to 8 wt % unless otherwise specifically treated.
When the GO is appropriately included in a film or a structure, strength thereof may be improved and suitable thermal conductivity may be provided, but treatment of the contained moisture may hinder properties.
In general, GO may be prepared in a form of graphene through a chemical reduction method (a hydrazine treatment and the like) and a thermal reduction method. Herein, reduced graphene is particularly referred to as reduced graphene oxide (RGO).
It is evidenced that a part of the oxidizing groups on the RGO surface is not sufficiently removed. Generally, oxygen content of the surface oxidizing groups is less than or equal to about 5 wt % relative to a carbon backbone, and thus graphene (RGO) of the present invention has an oxygen content of less than or equal to about 5 wt % because of the surface oxidizing groups relative to a carbon backbone.
A heterogeneous mixture of GO, and RGO and a conventional material has recently evoked active interest, and this may improve synergic effects between materials exceeding a limit of the conventional material. The heterogeneous mixture may be used in a high strength composite material and a fuel cell. In certain examples, a graphene-nanowire (semiconductor) hybrid structure where light energy is absorbed in a graphene conductive part and electron-hole pairs are generated (KR 10-2012-0092431 A), a hybrid composite manufacturing method including graphene sheet/carbon nanotube/a polymer nanoparticle (KR 10-2012-0053399 A), a method of manufacturing a positive electrode graphene material for a lithium rechargeable battery that is a hybrid material formed by adding an Fe precursor and a PO4 precursor (KR 10-2012-0035659 A), a method of manufacturing a graphene composite calcinated body having an excellent charge and discharge ratio by sintering graphene and a metal oxide particle in air (U.S. Pat. No. 8,257,867), a method of manufacturing a graphene-TiO2 hybrid material by mixing a TiO2 nanopowder with graphene at a high temperature and high pressure and reacting them (US 2012-0149554 A), a method of manufacturing a graphene ceramic composite (KR 2012-0039799A, and KR 2013-0014327A), and the like have been explored.
In addition, Publication Laid-open KR 10-2012-0039799 discloses a technology of improving coating properties of graphene itself by directly chemically bonding a ceramic precursor with a carboxyl group (—COOH) on the edge of GO to improve dispersion. For example, the graphene itself is coated on the edge of the GO through a chemical bond and may induce high electrical conductivity. However, the coating may be weak, since there is no binder between the GO and the coated graphene layer. Publication Laid-open KR10-2013-0014327 also discloses a method of making a graphene composite by mixing a salt type (e.g., chloride) ceramic precursor with graphene or a graphene oxide and then, calcinating the mixture at a high temperature. However, when the ceramic and the graphene have a directly chemical bond, the sheet-shaped structure of the graphene itself is broken, and the graphene may lose inherent properties.
This problem becomes severe, particularly when the ceramic is oxide, since a carbon component in the graphene is bonded with an oxygen component in the ceramic and released as gas such as CO and CO2, leaving a carbon residue as a particle. When an oxide ceramic precursor or oxide ceramic sol is calcinated with graphene, there is a similar problem to the above.
In addition, when graphene is reacted at a low temperature that the graphene has no reaction with oxygen without calcination in order to reduce the problem, there is a stripping problem due to severe deterioration of an interface bond between oxide ceramic and graphene. In other words, hydrophobicity of the graphene may resist against hydrophilicity of the ceramic.
In general, when a ceramic film is formed by coating ceramic sol and gelating it, the film may be destroyed due to evaporation of a solvent and an osmotic pressure during the drying. Meanwhile, when graphene is used to form a hybrid film, the hybrid film including the graphene in an appropriate concentration is actually difficult to form due to diffusion of the graphene into a solvent and prevention of drying (non-uniform drying since a graphene layer is positioned on the surface and prevents movement and diffusion of the solvent).
The hybrid film has difficulty in terms of commercial availability, since the graphene is hardly dispersed layer by layer in a solid-phase matrix to realize properties of a graphene-containing composite. In addition, a specific process example has not been provided yet.
Accordingly, in order to solve the problems, a method of reducing a metal precursor at room temperature to powder it and platting and sputtering graphene to manufacture composite powder or a composite layer has been suggested, and another method of using a polymer resin in an entire amount to avoid the fundamental problem of a ceramic composite is mostly used.
However, the above graphene composite materials are not sufficiently dispersed, and the polymer resin also has a drawback of sharply deteriorating thermal conductivity of the graphene and durability of the film.